Metal matrix composites exhibit superior mechanical properties over existing metal materials since the superior ductility of a metal matrix is combined with the high hardness, high elasticity and abrasion resistance of a reinforcing material. Therefore, the metal matrix composites are becoming more popular in the materials industry related to vehicles and aircraft.
Existing methods of manufacturing a metal matrix composite in which nanoparticles or nanofibers are dispersed can be divided into three methods, i.e. a liquid phase method (Noguchi T, Magario A, Fukazawa S, Mater Trans 2004; 45:602, Yanagi H, Kawai Y, Kita K, Japanese Journal of Applied Physics 2006: 45: L650-3), a solid phase method (Sie Chin Tjong, Advanced Engineering Materials, 9(8), 2007, pp. 639 to 652), and a spray forming method (S. K. Chaudhury, C. S. Sivaramakrishnan, S. C. Panigrahi, Journal of Materials Processing Technology 145 (2004) 385-390).
According to the solid phase method, it is difficult to uniformly disperse a reinforcing material in powders, but the reinforcing material is mixed on the surface of the powder. Consequently, a degree of the dispersion decreases and the characteristics of the composite deteriorate due to the segregation of the reinforcing material, for example, on the surface of the powder. In addition, since the reinforcing material present on the surface disturbs the bonding between powders when integrating the powders, it is difficult to manufacture a fine bulk material using this method. In addition, economic competitiveness caused by high manufacturing cost restricts the applicability of this method. Furthermore, the spray forming method is still at the research stage since some of powders may not be layered and controlling technologies for complex shapes and microstructures are required. This method is also restricted in terms of economic competitiveness due to high manufacturing cost.
The liquid phase method includes stir casting, squeeze casting, rheocasting, thixocasting, or the like. Among them, the stir casting is most popular in terms of simplicity, applicability, economic competiveness and productivity as a method of manufacturing a particle or discontinuous distribution-enhanced metal matrix composite.
However, in the stir casting method of the related art, simple mixing using the impeller and simple injection of a reinforcing material confront several problems to overcome, such as low wettability between the reinforcing material and the molten metal, the difference in the specific gravity between the molten metal and the reinforcing material, porous particles or nanofibers floating on a surface of the molten metal upon the casting, and the like. When a size of the reinforcing material decreases to a nanoscale, these problems become severe. Consequently, the manufacture of the composite becomes more difficult.
In the meantime, recently, interest on magnesium (Mg) is increasing as information technology (IT) devices are widely used. Mg is expected as a light-weight material in the transportation industry such as vehicles since the strength per unit weight of Mg is very high. However, Mg is vulnerable to corrosion, and when Mg is applied to cases of mobile phones or PCs or exterior parts of vehicles, surface treatment such as painting is required. Although the anodizing of Mg products is carried out by DOW17, HAE, or the like, the cost thereof is very high, so that several new surface treatment processes are being sought. However, a technology for fundamentally improving properties of the Mg material is still required.